Answering one outstanding question from the information I recorded yesterday, I went out this morning and logged the intake temperature again, this time with the full intake snorkel in place. The two sets of data from yesterday were made with the end of the intake removed and I thought that putting the system back to how it is supposed to be set up might make a difference. Hoping to see the intake temperatures drop 30 to 40 degrees Fahrenheit was probably too much to hope for, but it would have been nice to see some lower temperatures even if only by a small amount.
The chart below is from today and it doesn’t look much different from what I recorded yesterday.
Intake air temperature is a critical component of engine performance, colder air is denser which allows more air to enter each cylinder and more fuel to be burned, resulting in more power output. Conversely, if the intake air temperature climbs the air entering the engine becomes less dense and less fuel can be burned, decreasing power output. Increasing air temperature also moves the gasoline closer to the point where end gasses can become unstable leading to knock.
Keeping air intake temperatures low is a significant challenge on a turbocharged car, so much so that auto manufacturers frequently equip turbocharged cars with heat exchangers (aka intercoolers) purposefully built to lower the temperature of air entering the motor.
Today I wanted to investigate how the intake air temperatures changed during normal driving. With one air temperature sensor located between the turbocharger and the intercooler, and another located between the intercooler and the engine, I would be able to observe how the intercooler performed as well as the general change in temperature through the system.
Auber Air Temperature Sensor installed in charge pipe joining turbo and intercooler
Upon start up in the morning when the engine was cold all three temperature readings, the vehicle air intake temperature sensor, the instrument cluster displayed outside air temperature, and the Auber sensor in the charge pipe all read nearly the same, about 75 degF. Shown by the green line in the chart below. The horizontal axis is time in seconds and the vertical axis temperature in degrees Fahrenheit.
Cold Start Intake Air Temperature Rise Pre and Post Intercooler
What grabs my attention on this chart is how quickly the air temperature passing through the intake rises (Orange line is pre intercooler, blue line is post intercooler). The stock intercoolers are doing a good job initially, but the air exiting the turbocharger continues to steadily climb, even though the car is not being operated under any heavy loads. It isn’t until about 12 minutes (810 seconds) into the drive that the first heavy acceleration takes place. That acceleration has a momentary affect on the temperature causing an upward spike, but the air temperature quickly drops back to the pre-event level, but it continues to climb upward.
After just over 15 minutes of driving the temperature leaving the turbocharger at steady state driving is 50 degF higher than the air temperature entering the intake system.
After letting the car cool for the afternoon I went out for another drive approximately 6 hours after the first drive. The log for that period is shown below.
IAT Rise 6 Hours After Previous Drive
Not surprisingly the starting temperature is about 10 degF higher than the afternoon ambient temperatures since the car retains heat for a long time. As with the morning drive the intake air temperature starts to climb upward almost immediately. Once again, after approximately fifteen minutes of driving the temperature of the air exiting the turbocharger is about 50 degF higher than the starting temperature. The air at this point going into the intercooler was 145F without any boost pressure being applied, 60 degF above ambient temperature!
What’s Next:
It’s evident to me that the intercoolers, regardless of whether they are stock or aftermarket, do an excellent job of masking a potential source of improvement on this car.
The question I now have is, does the air passage through the turbocharger cause the dramatic rise? Could it be the close proximity of the turbo inlet pipe to the exhaust manifold?
Given the substantial affect that air temperature has on the engine’s performance, and the fact that even without boost pressurizing the charge air the temperature of the air going into the intercooler is 50-60 degrees above the ambient temperature, there appears to be an opportunity to improve the engines output if a solution can be found for bringing the intake air temperatures down.
As a follow up to the discussion about the Audi B5 S4 Stock Intercoolers here is a chart that illustrates just how effective all of the available intercoolers are at dropping the charge temperature.
Example of Temperature Drop achieved by Intercoolers
The chart above is a composite of several pieces of data to be illustrative of how effective the intercoolers are at achieving temperature drop in the charge air.
The measured compressor outlet temperature rise and IAT curve showing a 12 degF rise were recorded together on a stock boost (8 psi) pull. The curve showing a 28 degF IAT rise is from an S4 operating at 21 psi of boost pressure with stock IC’s.
On a typical stage 3 S4 operating with K04 size turbo’s it would not be unusual to see the intake air temperature increase by 28 degF by the end of a FATS pull. This is the 28 degF curve shown above.
Under similar conditions an ER intercooler would be expected to show a temperature rise in the neighborhood of 12 degF, shown by the second IAT curve above.
Typically an Intake Air Temperature comparison would show these two curves with an appropriate scaling that highlights how the Stock intercoolers allow IAT to rise more the twice what ER’s do – the impression that the divergent curves give is that there is a substantial performance difference between the two intercoolers.
When the turbo compressor outlet temperature is included, as above, some additional perspective is available. It becomes evident that both the Stock and ER intercoolers are dropping the charge temperature a significant amount – with a stock boost level the IC’s lower the charge temperature by roughly 100 degF.
But as I pointed out, the 28 degF IAT curve was obtained with Stock intercoolers at 21 psi of boost – a case that I have not yet measured the compressor outlet temperature for. Based on predicted outlet temperature the blue dotted line shows what the compressor outlet temperature might look like at 21 psi of boost.
Knowing that stock IC’s show a 28 degF delta and ER’s a 12 degF delta when recorded on a FATS pull at 21 psi of boost, the amount of temperature drop from the compressor outlet is around 170 degF for stock IC’s and 186 degF for ER’s. As the far right markings on the chart show, there is a big drop in temperature whether by the stock IC or the ER, and the difference in performance of the Stock IC and ER IC is relatively small in comparison to the amount of total temperature drop being achieved.
Bottom Line:
Both of these intercoolers are doing a good job at temperature drop. The aftermarket intercooler is able to achieve greater temperature drop than the stock IC, but in the context of the overall temperature drop of the charge air the differences between the two products is not that great.
